1. Field of the Invention
This invention relates to a projection device in which the exposure amount distribution due to the cosine 4th power rule of the lens is corrected even if the projection magnification is varied, and particularly to a projection device effective for copying apparatus of the slit exposure type which requires a uniform exposure amount in the lengthwise direction of the slit.
2. Description of the Prior Art
In variable magnification copying apparatus of the slit exposure type, there is a problem that exposure irregularity due to the so-called cosine 4th power rule to the angle of view is created in the lengthwise direction of the slit when the slit area of an original is projected onto a photosensitive medium by a lens.
To solve this problem, some apparatus use a light source such as a halogen lamp having a brightness distribution which corrects the cosine 4th power rule in the lengthwise direction, that is, such a distribution which increases the brightness in the marginal area as compared with the central area, whereas such apparatus have suffered from great irregularity in light distribution and required a special specification and this has led to a high cost of manufacture.
On the other hand, in copying apparatus using a light source such as a fluorescent lamp having a uniform brightness distribution, it is necessary that a stop plate for correcting the cosine 4th power rule be provided in the optical path. Among the apparatus provided with such a slit plate, there are ones in which a number of slit plates are put in and out during magnification change as shown in Japanese Laid-open Utility Model Application No. 84643/1980, ones in which an inclined slit plate is put in and out as shown in Japanese Laid-open Patent Application No. 91728/1978, ones in which a view field slit plate is moved near the photosensitive medium in the lengthwise direction of the slit as shown in Japanese Laid-open Patent Application No. 155523/1977, and ones in which a view field slit plate near the photosensitive medium is moved in a direction perpendicular to the lengthwise direction of the slit (scanning direction) relative to movement of the lens as shown in Japanese Laid-open Patent Application No. 54023/1979. However, these apparatus have suffered from the following disadvantages. As regards the apparatus in which a number of slit plates are put in and out, they have an increased number of mechanical parts and the change-over operation for each varied magnification or the shape adjustment of the slit plate during each magnification change is complicated, and this has led to a high cost.
As regards the apparatus in which an inclined slit plate is put in and out, correction irregularity can be well corrected while the mechanism for putting the slit plate in and out is complicated.
As regards the apparatus in which the slit is moved near the photosensitive medium, it is spatially difficult to mount a moving mechanism and the apparatus become bulky.